Long pause between shots while shooting brackets on multipule camera rig

I’m capturing HDR brackets ( 9 shots 1 ev ) on a multiple camera rig ( 4x Nikon D850 1x Sony a7r3 ) and there is a undesirable delay between each shot. Not sure if its because it has to process large RAW files or if I’m using the wrong USB cables. I am wondering how to get it to capture without any delay.